1. Field of the Invention
The present invention relates to a cam ring which can serve as an element of a zoom lens barrel. The present invention also relates to a zoom lens barrel which incorporates such a cam ring.
2. Description of the Related Art
In a typical conventional zoom lens barrel, a plurality of (e.g., two) movable lens groups are moved along an optical axis while changing the space therebetween in a predetermined moving manner to perform a zooming operation by rotation of a cam ring driven to rotate about the optical axis. Specifically, the zoom lens barrel is provided inside the cylindrical cam ring with a cylindrical linear guide ring, and is further provided inside the linear guide ring with lens support frames which hold and support the movable lens groups, respectively. A set of follower pins, or a set of cam followers, which project from each lens support frame, pass through an associated set of linear guide slots formed on the linear guide ring, respectively, to be guided linearly along the optical axis. At the same time, the heads (radially outer ends) of the set of follower pins are engaged in an associated set of cam grooves (bottomed cam grooves or cam through slots) formed on the cam ring, respectively. Due to engagement of the set of follower pins of each lens support frame with both the associated set of linear guide slots and the associated set of cam grooves, a rotation of the cam ring causes the movable lens groups to move along the optical axis while changing the space therebetween in a predetermined moving manner to perform a zooming operation.
In such a conventional zoom lens barrel, if the front and rear ends of each cam groove are respectively not open at the front and rear ends of the cam ring, and if the front and rear ends of each linear guide slot are not open at the front and rear ends of the linear guide ring, respectively, each set of follower pins cannot be respectively inserted into the associated set of cam grooves, with each set of follower pins remaining fixed to the associated lens support frame because the distance between the axial center of the lens support frame and the radially outer end of each follower pin is greater than the inner radius of the cam ring, i.e., the distance between the axial center of the cam ring and the inner peripheral surface of the cam ring. Due to this structure, when the set of follower pins of each lens support frame are respectively inserted into the associated set of cam grooves of the cam ring, firstly, the lens support frame to which the set of follower pins are not yet fixed needs to be inserted into the linear guide ring (which is inserted into the cam ring), and thereafter, the set of follower pins need to be respectively inserted into the associated set of cam grooves and the set of linear guide slots from the outside of the cam ring to be fixed to an outer peripheral surface of the lens support frame by a conventional fixing manner, e.g., by screw engagement: a male screw portion formed at the bottom end of each follower pin is installed in a female screw hole formed on the an outer peripheral surface of the lens support frame.
Such an operation making each set of follower pins engaged in the associated set of cam grooves is troublesome.
Moreover, each follower pin cannot be removed from the associated cam groove in a state where the follower pin remains fixed to lens support frame. In this case, firstly, each follower pin needs to be removed radially outwards from the lens support frame, e.g., each follower pin needs to be screwed off from the associated female screw hole on the lens support frame to be removed radially outwards therefrom. Thereafter, the lens support frame needs to be removed from the cam ring and the linear guide ring. This operation is also very troublesome.
Likewise, each follower pin cannot be inserted into or removed from the associated cam groove if each set of follower pins are formed integral with the associated lens support frame since the distance between the axial center of the lens support frame and the radially outer end of each follower pin is greater than the inner radius of the cam ring as mentioned above.
Furthermore, if a cylindrical member such as a stationary barrel of the zoom lens barrel is positioned around the cam ring, each follower pin cannot be inserted into or removed from the associated cam groove since each follower pin needs to be inserted into or removed from the associated cam groove from the outside of the cam ring to be fixed to an outer peripheral surface of the lens support frame as mentioned above.
On the other hand, in the case where the front and rear ends of each cam groove are respectively open at the front and rear ends of the cam ring while the front and rear ends of each linear guide slot are respectively open at the front and rear ends of the linear guide ring, the can ring cannot have a sufficient strength if each set of cams grooves on the cam ring are formed as a set of cam through slots. If each set of cams grooves formed on the cam ring are formed as a set of bottomed cam grooves, the wall thickness of the cam ring increases, i.e., the diameter of the cam ring increases to thereby increase the diameter of the zoom lens barrel.